Dual-curing adhesives have been in the center of interest for many years. Usually, curing by means of actinic radiation, in particular in the UV/VIS range, is combined with a second curing mechanism, which might be curing by using heat or moisture.
For example, one application of dual-curing adhesives is the fast fixation of fillet welds prior to the final curing step to keep structural components from shifting. In molding applications the surfaces can be rapidly light-sealed and the components easily handled without leakage of the molding mass. In such applications light curing is primarily a preliminary fixation. Furthermore, there are applications in which only adhesive residues are to be cured by means of a second curing mechanism, e.g. in undercuts not reached by the light.
So far a multitude of dual-curing mechanisms has been known. These mechanisms always comply with a certain type of chemical reactions and are thus subject to the restrictions related to the chemical substance class used.
The oldest type of dual-curing masses is dual-curing acrylate adhesives. As light curing of the acrylate groups is induced by photoinitiators, heat curing is performed using substances such as peroxides forming free radicals when heated. However, with acrylates there is the problem of oxygen inhibition, that is the surfaces remain moist as the growing chains at the surface of the adhesive are terminated by the oxygen radicals present in the air. Although this phenomenon can be avoided technically by inertization, e.g. by curing in a nitrogen atmosphere or by using hard radiation in the UV-B or UV-C range, the technical effort is correspondingly high.
Moreover, acrylate adhesives exhibit extreme shrinkage, poor temperature resistance and moderate moisture resistance. In particular the resistance to a combination of moisture and temperature, often tested in the so-called 85/85 test (85% humidity at 85° C., usually during 1000 hours), is a problem. This is why acrylate adhesives have not been used so far at a large scale, for example, in the electronics industry.
European patent EP 1 061 101 B1 discloses cationically dual-curing masses that are surface dry after curing. Although cationically dual-curing systems have gained some acceptance in the electronics field, for example in the production of smart card modules, the sensitivity of the systems on alkaline substrate surfaces, on which cationic polymerization is inhibited by chain growth termination, prevents a broader application of these masses.
German patent specification DE 10 2009 012 272 B4 describes light-curing acrylate adhesives comprising isocyanate-containing resins whose isocyanate groups are cross-linked via amine clusters in a second curing mechanism. However, it is an object of that patent specification to exclude liquid components in shadow regions. To this end distinctly reduced mechanical properties are accepted.
Dual-curing masses according to DE 10 2010 010 598 A1 also use moisture crosslinking albeit with α-silanes. Although here the mechanical properties of the cured masses are improved as compared to patent specification DE 10 2009 012 272 B4—for example, the mechanical properties will suffice to provide enough stability to shadow regions in bonded displays—the strength of the cured masses is not comparable to the strengths achieved with purely light-curing acrylate adhesives. In addition, these masses have the disadvantages mentioned before and already known with regard to acrylates.
In addition, light-fixable cyanoacrylates, generally known as superglue, and light-fixable silicones are commercially available. Again the disadvantages of the underlying chemistry, e.g. very poor moisture resistance of the cyanoacrylates or moderate adhesion properties of the silicones, prevail.
DE 10 2005 031 381 A1 discloses heat-curing masses in which (iso)cyanates are cured using amine-based latent curing agents. The possibility of mixing with epoxy resins is also described. However, the possibility of curing the masses by means of light is not disclosed.
Patent specification US 2012/0115972 describes a dual-curing system comprising a photoinitiator, an epoxy resin, a latent curing agent for the epoxy resin and a radical-curing resin. When irradiated with light, this system is supposed to result in extensive deep curing. Especially in the use of LCD displays, damage to the LCD display by uncured components in shadow regions is supposed to be avoided by a downstream heat-curing step. Thus, the properties of the dual-curing system correspond to a classic radically light-curing product. This is illustrated by the fact that the radical-curing resin is preferred to be present in a proportion of 40 to 90 percent.
For the general expertise regarding the adhesive systems known in the state of the art reference is made to the book “Formulierung von Kleb- and Dichtstoffen [Formulation of adhesives and sealing agents]” by B. Müller and W. Rath, 2004, published at Vincentz-Verlag Hanover.
Heat-curing single-component epoxy resins have been known to those skilled in the art for several decades. The curing of epoxy resins by using latent curing agents, mostly based on amines, results in masses that have been widely applied in almost all industrial fields due to their low shrinkage, good mechanical properties, chemical resistance and excellent thermal resistance. The properties of epoxy resins are, for example, described in the book “Epoxy Resins, Chemistry and Technology”, editor Clayton A. May, published at Marcel Dekker Verlag, New York, 2nd edition 1988.
However, functioning dual curing using heat-curing epoxy resins has not been known so far. Although meanwhile so-called “photolatent bases” containing protected amine groups have been described, which are available from the company BASF SE, for complete curing of the epoxy resin the amine would have to be released in a stoichiometric amount as compared to the epoxy groups. In addition, each amine would have to carry two reactive groups, i.e. it would have to be used as a diamine or as a primary amine containing two hydrogen atoms suitable for crosslinking. Thus, the use of these photolatent bases for light curing of epoxy resins is excluded.
In addition, several manufacturers offer so-called epoxy acrylates, that is hybrid compounds carrying both epoxy groups and acrylate groups. However, these epoxy acrylates either result—when the epoxy proportion of the masses is set high—in sticky surfaces after curing in the UV/VIS range or—when the acrylate proportion is set very high—in masses having the disadvantages known with regard to acrylates.